U.S. patent number 5,787,000 [Application Number 08/250,179] was granted by the patent office on 1998-07-28 for method and apparatus for scheduling work orders in a manufacturing process.
This patent grant is currently assigned to Lilly Software Associates, Inc.. Invention is credited to David V. Layne, Richard T. Lilly.
United States Patent |
5,787,000 |
Lilly , et al. |
July 28, 1998 |
Method and apparatus for scheduling work orders in a manufacturing
process
Abstract
A computerized system is provided for scheduling a plurality of
work orders in a manufacturing process. Each work order to be
scheduled specifies a set of operations to be performed using a
plurality of resources and materials. Data including resource
availability information for each resource used in the
manufacturing process, material availability information for each
material used in the manufacturing process, and work order
information is received and stored in a computer. The work order
information includes a release date for the work order, a want date
for the work order, operations information, and material
requirements information. The operations information includes the
identity and sequence of operations to be performed for the work
order, the identity of the resources needed to perform each
operation, a minimum resource capacity needed to perform each
operation, and the time needed to perform the operation. The
materials information includes the identity of the materials needed
to perform each operation and the quantity of each material needed
for the operation. Resource capacity and a start date/time and a
finish date/time are assigned to each operation based upon the
resource availability information, the material availability
information, and the work order information. The assigned resource
capacity, the assigned start date/time, and the assigned finish
date/time for each operation are displayed on a computer screen in
a graphical format.
Inventors: |
Lilly; Richard T. (Hampton
Falls, NH), Layne; David V. (Litchfield, NH) |
Assignee: |
Lilly Software Associates, Inc.
(Hampton, NH)
|
Family
ID: |
22946618 |
Appl.
No.: |
08/250,179 |
Filed: |
May 27, 1994 |
Current U.S.
Class: |
700/95;
700/100 |
Current CPC
Class: |
G06Q
10/06 (20130101) |
Current International
Class: |
G06Q
10/00 (20060101); G06F 019/00 (); G06G 007/64 ();
G06G 007/65 () |
Field of
Search: |
;364/468,478,401,402,403,468.02,468.03,468.05,468.06,468.07,468.13,468.18
;395/228,229 ;705/8 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ruggiero; Joseph
Assistant Examiner: Rao; Sheela S.
Attorney, Agent or Firm: Testa, Hurwitz & Thibeault
LLP
Claims
We claim:
1. A computerized method for scheduling a plurality of work orders
for manufacturing products in a manufacturing process, each work
order comprising a set of operations to be performed using a
plurality of resources and materials, the method comprising:
monitoring the availability of each resource used in the
manufacturing process;
monitoring the availability of each material used in the
manufacturing process;
inputting information for each work order to be scheduled, said
work order information comprising: a release date for the work
order; a want date for the work order; operations information
comprising the identity and sequence of operations to be performed,
the identity of the resources needed to perform each operation, a
minimum resource capacity needed to perform each operation, and the
time needed to perform each operation; and materials requirements
information comprising the identity of the materials needed to
perform each operation and the quantity of each material needed for
the operation;
assigning resource capacity and a start date/time and a finish
date/time to each operation based upon the resource availability,
the material availability, and the work order information, by
selecting a start date/time for each operation based upon a
determination that both the resource capacity and the material
required for that operation are concurrently available on such
start date/time, to provide a schedule; and
outputting the assigned resource capacity, the assigned start
date/time and the assigned finish date/time for each operation in a
graphical format.
2. The method according to claim 1, wherein said resource
availability monitoring step comprises monitoring for each
operation the identity of each resource used in the manufacturing
process, the available capacity for the resource, the dates when
said resource capacity is available, and the capacity for the
resource that has been assigned to previously scheduled
operations.
3. The method according to claim 1, wherein said material
availability monitoring step comprises monitoring for each
operation the identity of each material used in the manufacturing
process, the dates when the material becomes available, and the
quantity of the material available on each date.
4. The method according to claim 3, wherein said material
availability monitoring step further comprises monitoring an amount
of time necessary to obtain additional material.
5. The method according to claim 3 further comprising the step of
displaying on a computer screen selected material availability
information.
6. The method according to claim 1, wherein the work order
information further comprises at least one of the identity of the
product to be manufactured, the quantity of the product to be
manufactured, and a relative priority for each work order.
7. The method according to claim 6 further comprising the step of
displaying on a computer screen selected work order
information.
8. The method according to claim 7, wherein said selected work
order information is displayed in the form of an acyclic directed
graph.
9. The method according to claim 1, wherein the operations
information comprises at least one of the time needed to set up the
resources needed to perform each operation and the time needed to
run each operation per unit of output from the operation.
10. The method according to claim 1, wherein the operations
information further comprises at least one of the time needed to
transfer the output from each operation to the next operation in
the sequence and the minimum quantity of output units required to
be completed before beginning the next operation in the
sequence.
11. The method according to claim 1, wherein the operations
information further comprises at least one of the yield of output
units per unit of input for the operation and the quantity of
output units per unit of input that will be discarded as scrap.
12. The method according to claim 1, wherein the operations
information further comprises a maximum resource capacity that
could be used to perform the operation and said maximum resource
capacity is assigned to the operation if said maximum resource
capacity is available, and if said maximum resource capacity is not
available, then a resource capacity not less than the minimum
resource capacity is assigned to the operation.
13. The method according to claim 1, wherein the material
requirements information further comprises at least one of the
quantity of material that will be discarded as scrap during
issuance of material to the work order and the rate at which
material will be discarded as scrap during performance of the
operation for which the material is needed.
14. The method according to claim 1, wherein said resource
availability monitoring step comprises revising and storing
resource availability information based upon the assigned resource
capacity, the assigned start date/time, and the assigned finish
date/time.
15. The method according to claim 1, wherein the work order
information further comprises a relative priority for each work
order and the operations for each work order are scheduled in the
order of the work order priority.
16. The method according to claim 1, wherein the sequence of
operations comprises at least one subset of operations for
manufacturing an intermediate product, and at least two operations
for a single work order are scheduled concurrently.
17. The method according to claim 1, wherein more than one resource
are assigned to a single operation.
18. The method according to claim 1, wherein said material
availability monitoring step comprises:
(1) determining a first total quantity of a material needed to
perform all operations that have been scheduled prior to a given
date;
(2) determining a second total quantity of said material that has
been or will be received in inventory on or before said date;
and
(3) determining the difference between said first total quantity
and said second total quantity.
19. The method according to claim 1, wherein the step of selecting
a start date/time comprises:
(1) determining a proposed start date/time for the operation;
(2) for each material required for the operation,
(a) determining a first total quantity of the material needed to
perform all operations that have been scheduled prior to the
proposed start date/time;
(b) determining a second total quantity of the material that has
been or will be received in inventory on or before the proposed
start date/time; and
(c) computing the difference between said first total quantity and
said second total quantity; and
(3) comparing said difference to the material requirement for the
operation.
20. The method according to claim 19, further comprising:
(4) where said difference equals or exceeds the material
requirement for the operation, assigning said proposed start
date/time as the start date/time; and
(5) where said difference is less than the material requirement for
the operation, assigning a start date/time based upon said proposed
start date/time and the time required to obtain an additional
quantity of the material needed for the operation.
21. The method according to claim 1, wherein more than one schedule
for the same set of work order is output simultaneously.
22. The method according to claim 1, wherein the assigned resource
capacity, the assigned start dates/times, and the assigned finish
dates/times are output in the form of a bar graph.
23. The method according to claim 1 wherein the assigning step
further comprises traversing the operations of each work order in
at least one of the forward direction or the backward direction.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a computerized system for
scheduling work orders for manufacturing products in a
manufacturing process.
Prior to the introduction of computerized scheduling systems, the
scheduling of work orders in a manufacturing facility was performed
manually. The person responsible for determining the work schedule
manually assigned the work to be performed to specific dates. The
manual scheduling process involved determining what work has to be
performed and the time, materials, and resources (i.e., machines,
workers, tools, etc.) required to perform it, as well as other
information which influenced the schedule, such as which work was
more important than other work. This process is costly, very
tedious and often inaccurate. The frequency with which the
scheduling process must be performed contributes to the difficulty
of obtaining timely, accurate manual scheduling.
As commercial computer systems became available, manufacturers
began using computerized systems that assisted in the storage and
retrieval of information relevant to the scheduling of work in a
manufacturing facility. In the late 1950's and during the 1960's,
efforts were made to implement Material Requirements Planning or
MRP. MRP was limited in scope to dealing only with material
requirements (i.e., which parts or materials were needed and when).
These early computerized systems were not capable of scheduling
operations based upon the availability of materials or resource
capacity.
During the 1970's and 1980's, many commercially available systems
were created which provided more comprehensive MRP, and it was
eventually renamed Manufacturing Resource Planning. The new MRP
dealt with additional problems, such as forecasting, inventory
management, shop floor control, engineering, and change management.
Some of these systems dealt with the capacity of the plant to
perform the work. One such approach was called rough-cut capacity
planning and, as the name suggests, it provided a crude estimate of
the capacity needed to satisfy work requirements. Rough-cut
capacity planning determined the capacity needed, but was not
capable of scheduling work orders based on capacity limits.
Other systems for scheduling work orders in a manufacturing
facility relied on a method of scheduling that compared the total
hours of capacity to the total hours of load from operations. Such
systems were not capable of "event-oriented" scheduling, i.e.,
assigning a particular event or operation to a particular resource
for a particular time period.
It was not until the mid-1980's that a computerized event-oriented
scheduling system was developed that was capable of scheduling work
orders and the operations required to carry out the work orders,
taking into account the finite capacity of the various resources
needed for the manufacturing process, i.e., finite scheduling.
ProfitKey, a New Hampshire-based company, developed one of the
first such finite scheduling systems available to the market. Using
the ProfitKey scheduler, a manufacturer could determine the start
and finish date of work orders and their operations based upon
resource capacity and availability. A similar system was developed
by Lilly Software Associates, Inc. and marketed as part of the
VISUAL Manufacturing.TM. system beginning in early 1993.
However, neither the ProfitKey system nor the early 1993 version of
the VISUAL Manufacturing.TM. system was capable of taking the
availability of materials into account in determining the
scheduling of work orders. In addition, the ProfitKey system and
the early 1993 version of the VISUAL Manufacturing.TM. were both
limited to a single resource capacity calendar for all resources in
the system, and neither system was capable of generating multiple
schedules based on different assumptions regarding resource
capacity for individual resources. Further, the ProfitKey system
was not capable of scheduling multiple dissimilar resources
concurrently based upon user specifications.
Other scheduling systems include Microsoft Project.TM. and Norton's
Timeline from Symantec. These programs are capable of storing and
retrieving information about a project defined by the user using a
method consistent with Critical Path Method (CPM) theory. These
tools are designed to handle projects that have definable events
that are interdependent, similar to a single work order in a
manufacturing process. These systems are not, however, capable of
managing hundreds of work orders at a time as is typical in most
manufacturing plants.
No prior art system is capable of scheduling a plurality of work
orders or operations in a manufacturing facility, using a finite,
event-oriented scheduling process with the added feature of taking
into account the availability of materials that are used in the
manufacturing process.
OBJECTS OF THE INVENTION
Therefore, it is an object of the present invention to provide a
computerized system for performing accurate and timely scheduling
of work orders in a manufacturing facility.
It is a further object of the invention to provide a method and
means for scheduling work orders for manufacturing products in a
manufacturing process, wherein each operation in the work order is
assigned resource capacity, a start date/time, and a finish
date/time based upon the resource and material requirements of the
operation and the availability of resource capacity and materials
in the manufacturing facility.
It is still a further object of the invention to provide a method
and means for determining the best fit of the operations of each
work order in the schedule based upon both resource availability
and material availability.
It is still a further object of the invention to provide a
scheduling system which includes a method and means for defining
variable resource capacity as a function of the dates and times in
the calendar individually for each resource used in the
manufacturing facility.
It is still a further object of the invention to provide a
scheduling system which includes a method and means for creating
multiple schedules based on different resource capacity calendars
for comparison purposes.
It is still a further object of the invention to provide a
scheduling system which includes a method and means for scheduling
operations at more than one dissimilar resource simultaneously.
It is still a further object of the invention to provide a
scheduling system which includes a method and means for allowing
the user to specify a minimum and maximum resource capacity needed
to perform each operation being scheduled, in order to optimize
utilization of each resource in the manufacturing facility.
These and other objects and advantages of the invention will be
apparent from the following detailed description of the invention
and drawings.
SUMMARY OF THE INVENTION
In accordance with the present invention, a computerized system is
provided for scheduling a plurality of work orders in a
manufacturing process. Each work order to be scheduled specifies a
set of operations to be performed using a plurality of resources
and materials. The method of the invention includes receiving data
in a computer, the data including resource availability information
for each resource used in the manufacturing process, material
availability information for each material used in the
manufacturing process, and work order information. The work order
information includes a release date for the work order, a want date
for the work order, operations information, and material
requirements information. The operations information includes the
identity and sequence of operations to be performed for the work
order, the identity of the resources needed to perform each
operation, a minimum resource capacity needed to perform each
operation, and the time needed to perform each operation. The
materials information includes the identity of the materials needed
to perform each operation and the quantity of each material needed
for the operation. The method of the invention further includes the
steps of assigning resource capacity and a start date/time and a
finish date/time to each operation based upon the resource
availability information, the material availability information,
and the work order information. The method of the invention also
includes the step of displaying on a computer screen the assigned
resource capacity, the assigned start date/time, and the assigned
finish date/time for each operation in a graphical format.
The system of the invention includes means for receiving data in a
computer, the data including resource availability information for
each resource used in the manufacturing process, material
availability information for each material used in the
manufacturing process, and work order information. The work order
information includes a release date for the work order, a want date
for the work order, operations information, and material
requirements information. The system of the invention further
includes means for assigning resource capacity and a start
date/time and a finish date/time to each operation based upon the
resource availability information, the material availability
information, and the work order information. The system of the
invention also includes means for displaying on a computer screen
the assigned resource capacity, the assigned start date/time, and
the assigned finish date/time for each operation in a graphical
format.
DEFINITIONS
A "work order" order is a request to manufacture one or more
distinct parts in a manufacturing facility. Such parts may be
consumed either by the customer who ordered the parts or by other
work orders within the manufacturing facility, as in the case of a
subassembly.
A "part" is any distinct unit that can be manufactured as an end
product or an intermediate product or subassembly.
An "operation" is a step in the process for manufacturing a
distinct part according to a work order. Each operation in a work
order is sequenced so that it follows a preceding operation (or is
the first), and comes before a succeeding operation (or is the
last).
A "material" is any material or part that is consumed by the
manufacturing process or added to the part being manufactured
during the manufacturing process. A material may be a subassembly
which is an intermediate product used in the manufacturing process.
A material may be identified by an inventory part number.
A "resource" is something that is needed to perform an operation,
but which is not directly consumed in the performance of the
operation. Examples of a resource include workers, machines, and
tools. A resource may be internal or external to the manufacturing
facility. An external resource would be, for example, an outside
vendor or an outside service provider.
A "date/time" is a specification of a date (month, day, and year)
and a time of day.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the software components used to
implement a preferred embodiment of the scheduling system, as they
relate to each other;
FIG. 2 is a block diagram of particular routines within the
scheduling system and their relationship to each other;
FIG. 3 illustrates a preferred embodiment of a screen display
showing a multiple level work order;
FIG. 4 is a graphical representation of a multiple level work
order, omitting the material requirements;
FIG. 5 is a graphical representation of a capacity vector;
FIG. 6 is a graphical representation of a load vector; and
FIG. 7 illustrates a preferred embodiment of a screen display
showing a schedule generated by the scheduling system of the
invention.
FIGS. 8-12 are flow charts showing the steps for implementing the
scheduling system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The scheduling system of the present invention may be used in any
manufacturing facility. The system may be most useful in a facility
for manufacturing discrete products, such as a facility that
manufactures products according to individualized customer
specifications.
The scheduling system may be used to schedule all work orders that
have been accepted by the manufacturer. The system may also be used
to determine a proposed schedule and/or a proposed delivery date
for a potential work order in response to an inquiry from a
customer or other interested person.
The scheduling system of the present invention may be implemented
using a variety of personal computers. According to a preferred
embodiment of the invention, the scheduling system is implemented
using an IBM-compatible computer having not less than 8 megabytes
of random access memory (RAM), a 486DX or higher Intel.TM.
microprocessor running at 33 megahertz or higher, and sufficient
hard disk space to store the software programs used to implement
the invention, the output from the scheduling program, as well as
the input data such as the resource availability information,
material availability information, work order information,
operations information, and material requirements information.
The scheduling system of the present invention may be implemented
using a variety of programming languages and operating systems.
According to a preferred embodiment of the invention, the
scheduling system is implemented as a Windows Dynamic Link Library
using Gupta Corporation's SQLWindows.TM. Client-Server Application
Development System and Microsoft Visual C++.TM., both of which run
on a Microsoft.RTM. Windows.TM. Version 3.1-based IBM-compatible
computer system. FIG. 1 illustrates in a graphical format the
relationship between the software components used to implement a
preferred embodiment of the scheduling system of the invention. As
illustrated in FIG. 1, a preferred embodiment of the invention
interfaces with a manufacturing information system, which exchanges
certain data with the scheduling system, such as material
availability information.
The capabilities and implementation of the aforementioned
programming languages and operating systems are explained in the
following texts and manuals, which are hereby incorporated by
reference: SQLWindows Technical Reference Manual, Volumes 1 and 2,
Gupta Corporation, 20-1205-1003, 20-1206-1001; SQLWindows Function
Reference Manual, Volumes 1 and 2, Gupta Corporation, 20-2215-1003,
20-2217-1001; SQLBase Language Reference Manual, Gupta Corporation,
20-2107-1002; SQLBase C Application Programming Interface Reference
Manual, Gupta Corporation, 20-2111-1002; Microsoft Windows 3.1
Programmer's Reference, Volumes 1, 2, 3 and 4, Microsoft
Corporation; Microsoft Visual C++ Programmer's Guide, Microsoft
Corporation; and Microsoft Visual C++ Reference, Volumes 1 and 2,
Microsoft Corporation. Additional references which may be helpful
are listed bibliographically in the foregoing texts and
manuals.
According to the scheduling system of the present invention,
certain data required to schedule a work order is received in a
computer. This data includes resource availability information,
material availability information, work order information,
operations information, and material requirements information. Each
of these types of data is described in detail below:
"Resource availability information" includes the identity of each
resource used in the manufacturing process, a calendar specifying
the quantity of each resource that is available for use at a
particular date/time independent of previously scheduled operations
(the "resource capacity calendar"), and the capacity for each
resource that has been assigned to previously scheduled operations
(also referred to as "load"). The resource availability information
is updated each time that an operation is scheduled by the system
in order to reflect the load on each resource imposed by the
scheduled operation.
"Material availability information" includes the identity of each
material used in the manufacturing process, the dates when each
material is available, and the quantity of each material available
on each date. The material availability information may also
include the lead time necessary to obtain an additional quantity of
each material. The material availability information is updated
each time that an operation is scheduled by the system in order to
reflect the material demand in the time period the material is
needed for the scheduled operation. In a preferred embodiment of
the invention, selected material availability information is
displayed on a computer screen.
"Work order information" includes the release date for the work
order (i.e., when work should commence) and the want date for the
work order (i.e., when work must be complete). Work order
information also includes certain operations information and
material requirements information. Work order information may also
include the identity of the part to be manufactured, the quantity
of the part to be manufactured, and a relative priority for the
work to be performed. In a preferred embodiment of the invention,
selected work order information is displayed on a computer screen,
preferably in the form of an acyclic directed graph. FIG. 3
illustrates a preferred embodiment of a screen display showing
selected work order information.
"Operations information" includes the identity of the operations
defined by a work order, the sequence in which the operations are
to be performed, the identity of the resource(s) at which each
operation is to be performed, and a minimum resource capacity
needed to perform each operation. The resources identified for each
operation include a main resource, and optionally two auxiliary
resources, namely a setup resource and a run resource. The
"operations information" also includes the period of time needed to
perform the operation, which may include the period of time need to
set up the resource(s) needed to perform the operation ("setup
time") and the period of time needed to run the operation,
preferably expressed per unit of output from the operation or as a
number of output units in a given period of time ("run time"). The
"operations information" may also include the scrap or yield from
the operation, preferably expressed as the number of output units
from the operation per unit of input in percent; a fixed quantity
of scrap that will be discarded from the operation; a minimum
quantity of output units required to be completed before beginning
the next operation; the period of time required to physically
transfer the output units to the next succeeding operation
("transfer time"); and/or a maximum resource capacity that could be
used to perform the operation, if such capacity were available.
The sequence of operations defined by a work order may be single or
multiple level. A single level work order has a linear sequence of
operations. A multiple level work order has a branched sequence of
operations, whereby each branch of the sequence contains a subset
of operations for manufacturing an intermediate product or
subassembly that is used to manufacture the final product. FIG. 3
shows a graphical illustration of a multiple level work order,
including end product 31, material requirement 32, operation 33,
and intermediate products (or subassemblies) 34 and 35. Each subset
of operations is referred to as a "leg" of the work order. For
example, the work order shown in FIG. 3 has legs 36 and 37. For
multiple level work orders, the "operations information" includes,
as part of the sequence of operations, the leg of the work order to
which each operation belongs. In a preferred embodiment of the
scheduling system, operations that belong to separate legs of a
multiple level work order may be scheduled concurrently. For
example, FIG. 4 shows a graphical representation of a multilevel
work order, omitting the material requirements for the work order.
The top part of FIG. 4 illustrates the concurrent scheduling of
operations in a multiple level work order. In addition, even with a
single level work order, the user may specify that two or more
operations be scheduled concurrently.
"Material requirements information" includes the identity of the
materials needed for an operation and the quantity of the material
needed for an operation. The quantity of the materials needed for
an operation is preferably expressed either per unit of input to
the operation or per unit of output from the operation. The
materials information may also include a scrap rate (in addition to
the scrap imposed by the operation definition), a fixed quantity of
scrap (in addition to the scrap imposed by the operation
definition), the dimensions of a material part needed for an
operation, and/or the unit of measure for the quantity expression
(which may need to be converted to another unit of measure).
Ultimately, the various quantity expressions are reduced to a
specific quantity, in the stock keeping unit of measure, of the
part which is required.
The scheduling system of the present invention assigns resource
capacity, a start date/time, and a finish date/time to each
operation in the work order being scheduled based upon the resource
availability information, the material availability information,
and the work order information. The system schedules an operation
for a date and time when both the necessary resource capacity is
available and the necessary materials are available. The preferred
means for implementing the assignment of resource capacity, start
date/time, and finish date/time is described in detail below:
Resources can have one or more units of capacity. Each unit of
capacity corresponds to a worker, machine, tool, or other item or
device which performs work on an operation. According to the
invention, the system user can define the capacity of each resource
that is used in the manufacturing process as a function of the
dates and times in a calendar. This is called the "resource
capacity calendar." If, for example, the resource is machine
operators, the user may define the capacity as being one or more
machine operators for a given time period. The "resource capacity
calendar" is independent of load (i.e., the capacity that has been
assigned to scheduled operations). When scheduling an operation,
however, the system takes into account both the resource capacity
calendar and the load from previously scheduled operations.
As part of the work order, the user specifies the resource capacity
that is required to perform a particular operation. If more than
one person or machine is required to work on the same operation at
the same time, a minimum resource capacity of more than one unit of
capacity may be specified for a given operation. If it would be
desirable, but not necessary, for more than one person or machine
to work on the same operation at the same time, then a maximum
resource capacity may also be specified. In that event, the maximum
resource capacity will be assigned to the operation, if such
capacity is available. Thus, by specifying a maximum resource
capacity, the user may shorten the overall time required to perform
an operation by utilizing more than the minimum capacity.
According to a preferred embodiment of the scheduling system of the
invention, the resource capacity calendar is divided into distinct
units of time called notches. A notch is a specific length of time,
for example, ten minutes or six notches per hour. The system uses a
"capacity vector" to represent the time frame of the resource
capacity calendar. FIG. 5 shows a graphical representation of a
capacity vector. The capacity vector is divided into notches 51.
For example, a two-year time frame contains 17,520 hours or 105,120
notches. Associated with each notch in the capacity vector is a
designation of the capacity of the resource that is available
during the time period represented by the notch, i.e., the number
of simultaneous operations that may be scheduled in the notch. A
constant, for example, 255 (the maximum integer represented by a
single byte), designates a shutdown notch. A shutdown notch
represents a time period during which the resource is not being
operated, for example, because the manufacturing plant is closed.
All other values designate a running notch. A running notch can be
available or unavailable. An available notch is one which has
available capacity, and an unavailable notch is one which has no
available capacity.
For example, FIG. 5 shows a portion of a capacity vector for
machine operators. The first six notches 52 are shutdown notches,
indicating that the machine operators are not working during the
hour represented by these notches. The next eighteen notches 53 in
the capacity vector are running notches. Twelve of these notches 54
each have five units of available capacity, and six of these
notches 55 each have three units of available capacity. In other
words, there are five machine operators available during the two
hours represented by the middle twelve notches in the capacity
vector, and three machine operators available during the one hour
represented by the last six notches in the capacity vector.
According to a preferred embodiment of the scheduling system of the
invention, the capacity vector does not reflect the capacity that
has been assigned to previously scheduled operations. Rather, the
system records usage of capacity or load in the form of a "load
vector" which includes a list of operations that have been
scheduled at each resource. FIG. 6 shows a graphical representation
of a load vector. Each number 61 along the y-axis in FIG. 6
indicates a particular unit of capacity, e.g., a particular machine
operator. Time is represented along the x-axis. Each bar 62 in FIG.
6 represents the time assigned to scheduled operations, and the
arrows 63 between each bar 62 represent unassigned time. The space
to the left of the last bar 62 for a given unit of capacity also
represents unassigned time. Thus, the left end 64 of each bar 62
represents the start date/time for an operation, while the right
end 65 of each bar 62 represents the finish date/time for an
operation.
According to a preferred embodiment of the scheduling system of the
invention, an operation is scheduled in a given set of notches only
if all notches in the set are both adjacent and available. Two
notches are considered adjacent if they are separated by zero or
more shutdown notches, but not if they are separated by an
unavailable notch. A notch is available if there is at least one
unit of capacity which has not been previously assigned to another
operation. In order to determine whether a notch is available, the
system consults both the capacity vector (FIG. 5) and the load
vector (FIG. 6). Units of capacity to which no operation has been
assigned are available for assignment to subsequently scheduled
operations requiring that resource.
In addition to resource capacity, the scheduling system of the
present invention also checks material availability when scheduling
an operation. According to a preferred embodiment of the scheduling
system of the present invention, material availability is expressed
in terms of supply and demand for each material used in the
manufacturing process. Supply is an expression of when a material
will be received in inventory, and how much or how many, in terms
of the stock unit of measure, will be received. Thus, supply is a
list of quantities and dates listed, for example, in sequence by
date. Demand is an expression of when a material is needed to
perform scheduled operations, and how much or how many, in terms of
the stock unit of measure, is/are needed. Thus, demand is a list of
quantities and dates listed, for example, in sequence by date.
Material availability is determined by netting the demand and
supply lists. In a preferred embodiment of the scheduling system,
the system first determines a proposed start date for an operation.
The system then nets the supply and demand for the material(s)
required for the operation in order to determine the quantity of
the material(s) available on the proposed start date/time. If this
quantity satisfies the material requirement for the operation, then
the operation may be scheduled for the proposed start date/time.
If, on the other hand, this quantity does not satisfy the material
requirement for the operation, then additional material must be
obtained, and the start date/time may be rescheduled, taking into
account the lead time necessary to obtain an additional quantity of
the material. If the lead time is greater than the difference
between the current date/time and the proposed start date/time,
then the system adds the excess to the proposed start date/time to
determine the start date/time. If, on the other hand, the lead time
is less than or equal to the difference between the current
date/time and the proposed start date/time, then the operation is
scheduled for the proposed start date/time.
According to a preferred embodiment of the invention, the
scheduling system may operate in at least two different modes:
sequential scheduling and global scheduling. In the sequential
scheduling mode, the system schedules each work order as it is
entered in the system, leaving the assigned resource capacities,
the assigned start dates/times, and the assigned finish dates/times
for all previously entered work orders in place. In this mode, each
successively entered order is de facto given the lowest priority.
In the global scheduling mode, on the other hand, the system
reschedules all previously entered work orders in order of priority
each time that a new work order is entered in the system. All work
orders are automatically assigned the same priority when entered,
unless the user sets a different priority. Thus, in the global
scheduling mode, all work orders in the system are rescheduled in
the order of: (1) work order want date, if no work order priority
is specified; or (2) work order priority and want date within the
same priority level, if a work order priority is specified. The
sequence in which work orders are scheduled ultimately determines
the schedule. Available capacity is assigned to the first work
order in the sequence. Any remaining capacity is assigned to the
second work order in the sequence, and so on. Thus, the global
scheduling mode results in the optimum schedule because work orders
are scheduled in order of want date and/or priority.
According to a preferred embodiment of the scheduling system of the
invention, after the sequence in which work orders will be
scheduled is determined, the system processes each work order one
at a time in the determined sequence. Within each work order, the
operations are scheduled in the backward direction, starting with
the last or latest-in-time operation and/or in the forward
direction, starting with the first or earliest-in-time operation.
For example, the sequence of operations in the forward direction
for the multiple level work order shown in FIG. 4 would be:
10(4), 10(2), 10(1), 20(1), 10(3), 10(0), 20(0), 30(0), 40(0),
and
the sequence of operations in the backward direction would be:
40(0), 30(0), 20(0), 10(0), 10(3), 20(1), 10(1), 10(2), 10(4),
where the first numeral designates the operation and the numeral in
parentheses designates the leg of the work order to which the
operation belongs.
According to a preferred embodiment of the invention, the
scheduling system initially schedules all operations of a work
order in the backward direction. If backward scheduling results in
a schedule in which no operations are scheduled prior to the
current date, then the resulting schedule is saved and displayed.
If the result of backward scheduling exceeds the current date, then
forward scheduling is performed. After scheduling the operations of
a work order in the forward direction, the system differentiates
between "determinate" and "non-determinate" operations and
reschedules the non-determinate operations, if possible. FIG. 2 is
a block diagram illustrating these scheduling routines and their
relationship to each other.
In a single level work order, all operations are determinate. A
multiple level work order, on the other hand, has both determinate
and non-determinate operations. A determinate operation is one that
is on the path of operations which define the longest series of
operations, in terms of the length of time from the earliest start
date/time to the latest finish date/time, in view of the start
dates/times and finish dates/times of all operations in the work
order as presently scheduled. This is similar to the concept of a
critical path. A sequence of operations may become determinant when
one or more operations in the sequence is delayed by the
unavailability of resource capacity. In a preferred embodiment of
the scheduling system of the invention, after forward scheduling,
the system reschedules all non-determinate operations, assigning
the latest possible start date/time to each operation. The purpose
of this rescheduling is to achieve an optimum schedule which
minimizes work in process.
The logical steps contained in a computer program for implementing
a preferred embodiment of the scheduling system of the invention
are described in detail below with reference to the flow charts set
forth in FIGS. 8-12:
SCHEDULING A SINGLE WORK ORDER (FIG. 8)
1. LOAD THE WORK ORDER INTO MEMORY (FIG. 8, BLOCK 81)
2. IF USER SPECIFIES ONLY FORWARD SCHEDULING, SKIP TO STEP 5 (FIG.
8, BLOCK 82)
3. TRAVERSE THE OPERATIONS OF THE WORK ORDER IN THE BACKWARD
DIRECTION (FIG. 8, BLOCK 83) (see further explanation below)
4. IF STEP 3 RESULTS IN A SCHEDULE IN WHICH NO OPERATION IS
SCHEDULED PRIOR TO THE CURRENT DATE, THEN GO TO STEP 8 (FIG. 8,
BLOCK 84)
5. TRAVERSE THE OPERATIONS OF THE WORK ORDER IN THE FORWARD
DIRECTION (FIG. 8, BLOCK 85) (see further explanation below)
6. DETERMINE WHICH OPERATIONS OF THE WORK ORDER ARE
NON-DETERMINANT; TRAVERSE THE OPERATIONS OF THE WORK ORDER IN THE
BACKWARD DIRECTION AND RESCHEDULE ONLY THE NON-DETERMINANT
OPERATIONS (FIG. 8, BLOCK 86)
7. SAVE THE RESULTS (FIG. 8, BLOCK 87)
STEP 3 (BACKWARD SCHEDULING) CAN BE BROKEN DOWN INTO THE FOLLOWING
STEPS (FIG. 9)
a. LOCATE AND PROCESS EACH OPERATION, STARTING WITH THE LAST
OPERATION AND ENDING WITH THE FIRST OPERATION, EXECUTING THE
FOLLOWING STEPS FOR EACH ONE (FIG. 9, BLOCK 91):
b. CHECK THAT THE RESOURCE CAPACITY INFORMATION HAS BEEN LOADED FOR
EACH RESOURCE REQUIRED FOR THE OPERATION; IF NOT, READ THE CAPACITY
VECTOR INTO MEMORY AND INITIALIZE THE LOAD VECTOR (FIG. 9, BLOCKS
92, 93)
c. DETERMINE A PROPOSED FINISH DATE/TIME FOR THE OPERATION (FIG. 9,
BLOCK 94)
(1) for the last operation, the proposed finish date is the want
date of the work order
(2) for each preceding operation, the proposed finish date is the
start date of the succeeding operation, unless a minimum move
quantity is specified for the operation
(3) if a minimum move quantity is specified for the operation, then
the proposed finish date is the earlier of (a) the start date for
the succeeding operation plus the time necessary to complete the
operation for the minimum move quantity and (b) the finish date for
the succeeding operation less the time necessary to complete the
operation for the minimum move quantity
d. DETERMINE A PROPOSED START DATE/TIME FOR THE OPERATION (FIG. 9,
BLOCK 95)
(1) determine whether there is sufficient available capacity to
complete the operation during an uninterrupted time period,
consulting both the capacity vector(s) and the load vector(s) for
the resource(s) needed to perform the operation
(2) in order for an operation to be scheduled, a sequence of one or
more notches must be available which have no intervening running
notches with insufficient capacity to satisfy the minimum capacity
specified by the work order for the main resource for the
operation
(3) each operation has, optionally, two auxiliary resources for the
setup and run phases of the operation; if specified, the setup
resource must be available in the same notch as the main resource
during the setup phase; also, if specified, the run resource must
be available in the same notch as the main resource during the run
phase
e. DETERMINE WHETHER THE MATERIAL NEEDED FOR THE OPERATION IS
AVAILABLE (FIG. 9, BLOCK 96)
(1) by netting supply and demand, determine whether sufficient
material is available on the proposed start date to satisfy the
material requirement for the operation
(2) if sufficient material is available, then the proposed start
date becomes the start date
(3) if sufficient material is not available, backward scheduling
(STEP 3) is aborted and forward scheduling (STEP 5) must be
performed
STEP 5 (FORWARD SCHEDULING) CAN BE BROKEN DOWN INTO THE FOLLOWING
STEPS (FIG. 10)
a. LOCATE AND PROCESS EACH OPERATION, STARTING WITH THE FIRST
OPERATION AND ENDING WITH THE LAST OPERATION, EXECUTING THE
FOLLOWING STEPS FOR EACH ONE (FIG. 10, BLOCK 101):
b. CHECK THAT THE RESOURCE CAPACITY INFORMATION HAS BEEN LOADED FOR
EACH RESOURCE REQUIRED FOR THE OPERATION; IF NOT, READ THE CAPACITY
VECTOR INTO MEMORY AND INITIALIZE THE LOAD VECTOR (FIG. 10, BLOCKS
102, 103)
c. DETERMINE A PROPOSED START DATE/TIME FOR THE OPERATION (FIG. 10,
BLOCK 104)
(1) for the first operation, the proposed start date is the release
date for the work order or the current date, whichever is later
(2) for each succeeding operation, the proposed start date is the
finish date of the preceding operation, unless a minimum move
quantity is specified for the preceding operation
(3) if a minimum move quantity is specified for the preceding
operation (either in the same leg or in an immediately preceding
leg of the work order), then the proposed start date is determined
by the following steps:
(a) determine a proposed start date by adding the amount of time
necessary to complete the preceding operation for the minimum move
quantity to the start date for the preceding operation
(b) determine a calculated finish date for the current operation by
adding the run time for the current operation to the proposed start
date
(c) compare the calculated finish date for the current operation
with the finish date (determined in step e.) for the preceding
operation
(d) if the calculated finish date for the current operation is
earlier than the finish date for the preceding operation, then
subtract the run time for the current operation from the finish
date for the preceding operation to determine the proposed start
date; otherwise, use the proposed start date determined in step
(3)(a)
d. DETERMINE WHETHER THE MATERIAL NEEDED FOR THE OPERATION IS
AVAILABLE (FIG. 10, BLOCKS 105, 106)
(1) by netting supply and demand, determine whether sufficient
material is available on the proposed start date to satisfy the
material requirement for the operation
(2) if sufficient material is available, then the proposed start
date becomes the start date
(3) if sufficient material is not available, then load the lead
time for obtaining additional material into memory
(4) if the lead time is greater than the difference between the
current date and the proposed start date, then add the excess to
the proposed start date to determine the start date
(5) if the lead time is less than or equal to the difference
between the current date and the proposed start date, then the
proposed start date becomes the start date
e. DETERMINE A FINISH DATE/TIME FOR THE OPERATION (FIG. 10, BLOCK
107)
(1) determine whether there is sufficient available capacity to
complete the operation during an uninterrupted time period,
consulting both the capacity vector(s) and the load vector(s) for
the resource(s) needed to perform the operation
(2) in order for an operation to be scheduled, a sequence of one or
more notches must be available which have no intervening running
notches with insufficient capacity to satisfy the minimum capacity
specified by the work order for the main resource for the
operation
(3) each operation has, optionally, two auxiliary resources for the
setup and run phases of the operation; if specified, the setup
resource must be available in the same notches as the main resource
during the setup phase; also, if specified, the run resource must
be available in the same notches as the main resource during the
run phase
STEP 7 IS BROKEN DOWN INTO THE FOLLOWING STEPS (FIG. 11)
a. LOCATE AND PROCESS THE NON-DETERMINANT OPERATIONS, STARTING WITH
THE LAST NON-DETERMINANT OPERATION AND ENDING WITH THE FIRST,
EXECUTING THE FOLLOWING STEP FOR EACH ONE (FIG. 11, BLOCKS 111,
112):
(1) a non-determinant operation is one that is not on the
determinant (or critical) path.
b. IF POSSIBLE, SHIFT THE OPERATION TO THE LATEST AVAILABLE NOTCH
USING THE SAME UNIT OF CAPACITY (FIG. 11, BLOCKS 113, 114)
SCHEDULING A SINGLE OPERATION
The process of determining a start date (STEP 3.d.) or a finish
date (STEP 5.e.) for an operation will now be explained in greater
detail.
The process of determining a start date or finish date for an
operation assumes the availability, in the memory of the computer,
of a capacity vector and a load vector for the main resource, the
setup resource, and the run resource; the proposed finish date
(STEP 3) or proposed start date (STEP 5) for the operation; the
units of capacity required at each resource; and the run time,
setup time, and transfer time, in minutes, for the operation.
DETERMINING A START DATE/TIME WHEN TRAVERSING THE WORK ORDER IN THE
BACKWARD DIRECTION--STEP 3.d CAN BE BROKEN DOWN INTO THE FOLLOWING
STEPS (FIG. 12)
1. CONVERT THE PROPOSED FINISH DATE/TIME TO A RELATIVE NOTCH (i.e.,
THE nth notch from beginning of capacity vector, which starts at
midnight on the current date)--THIS BECOMES THE "CURRENT NOTCH;"
CONVERT TIME PERIODS TO NOTCHES (FIG. 12, BLOCK 121)
2. STARTING FROM THE "CURRENT NOTCH," SKIP NOTCHES FOR THE TRANSFER
TIME--THIS BECOMES THE NEW "CURRENT NOTCH" (FIG. 12, BLOCK 122)
3. SEARCH BACKWARD FROM THE "CURRENT NOTCH" FOR THE FIRST RUNNING
(NON-SHUTDOWN) NOTCH FOR THE MAIN RESOURCE; NOTE THIS NOTCH AS THE
"COULD END NOTCH"--THIS BECOMES THE NEW "CURRENT NOTCH" (FIG. 12,
BLOCKS 123, 124, 125)
4. SEARCH BACKWARD FROM THE "CURRENT NOTCH" FOR THE FIRST AVAILABLE
RUNNING NOTCH FOR ALL RESOURCES REQUIRED; NOTE THIS NOTCH AS THE
"ACTUAL END NOTCH"--THIS BECOMES THE NEW "CURRENT NOTCH" (FIG. 12,
BLOCKS 126, 127, 128)
5. SEARCH BACKWARD FROM THE "CURRENT NOTCH" COUNTING ALL AVAILABLE
NOTCHES FOR THE MAIN RESOURCE; VERIFY THAT THE SAME NOTCH IS
AVAILABLE FOR THE SETUP AND RUN RESOURCES, IF APPLICABLE; WHEN THE
NUMBER OF NOTCHES SPECIFIED BY THE SETUP TIME AND RUN TIME MATCHES
THE NUMBER OF NOTCHES ASSIGNABLE, END THE SEARCH; NOTE THIS NOTCH
AS THE "ACTUAL START NOTCH" (FIG. 12, BLOCKS 129, 130, 131,
132)
6. CONVERT ALL NOTCHES TO DATES/TIMES (FIG. 12, BLOCK 133)
7. RETURN RESULT OF SCHEDULE (FIG. 12, BLOCK 134)
8. IF AT ANY TIME THE BOUNDARIES OF THE SCHEDULE (i.e., the current
date) ARE EXCEEDED, ABORT THE SCHEDULING ROUTINE AND REPORT THE
APPROPRIATE RESULT
DETERMINING A FINISH DATE/TIME WHEN TRAVERSING THE WORK ORDER IN
THE FORWARD DIRECTION--STEP 5.e CAN BE BROKEN DOWN INTO THE
FOLLOWING STEPS
1. CONVERT PROPOSED START DATE/TIME TO A RELATIVE NOTCH (i.e., the
n.sup.th notch from beginning of capacity vector, which starts at
midnight on the current date)--THIS BECOMES THE "CURRENT NOTCH;"
CONVERT TIME PERIODS TO NOTCHES
2. SEARCH FORWARD FROM THE "CURRENT NOTCH" FOR THE FIRST RUNNING
(NON-SHUTDOWN) NOTCH FOR THE MAIN RESOURCE; NOTE THIS NOTCH AS THE
"COULD START NOTCH"--THIS BECOMES THE NEW "CURRENT NOTCH"
3. SEARCH FORWARD FROM THE "CURRENT NOTCH" FOR THE FIRST AVAILABLE
RUNNING NOTCH FOR ALL RESOURCES REQUIRED; NOTE THIS NOTCH AS THE
"ACTUAL START NOTCH"--THIS BECOMES THE NEW "CURRENT NOTCH"
4. SEARCH FORWARD FROM THE "CURRENT NOTCH" COUNTING ALL AVAILABLE
NOTCHES FOR THE MAIN RESOURCE; VERIFY THAT THE SAME NOTCH IS
AVAILABLE FOR THE SETUP AND RUN RESOURCES, IF APPLICABLE; WHEN THE
NUMBER OF NOTCHES SPECIFIED BY THE SETUP TIME AND RUN TIME MATCHES
THE NUMBER OF NOTCHES ASSIGNABLE, END THE SEARCH
5. SKIP NOTCHES FOR THE TRANSFER TIME; NOTE THIS NOTCH AS THE
"ACTUAL END NOTCH"
6. CONVERT ALL NOTCHES TO DATES/TIMES
7. RETURN RESULT OF SCHEDULE
8. IF AT ANY TIME THE BOUNDARIES OF THE SCHEDULE ARE EXCEEDED,
ABORT THE SCHEDULING ROUTINE AND REPORT THE APPROPRIATE RESULT
DETERMINATION OF RUN TIME AND MATERIAL REQUIREMENTS FOR EACH
OPERATION
As explained above, the run time, the quantity of materials needed
for each operation, and the scrap or yield rates are preferably
expressed as a function of the number of units of input to the
operation or the number of units of output from the operation.
According to a preferred embodiment of the scheduling system of the
invention, prior to scheduling the operations of a work order, the
run time and material requirement for each operation in the work
order are calculated in absolute terms using the input or output
quantities for each operation. Based upon the quantity specified by
the work order and the scrap or yield definitions, the input or
output quantities and material requirement for each operation are
determined by the following process prior to scheduling:
1. TRAVERSE THE OPERATIONS OF THE WORK ORDER IN THE BACKWARD
DIRECTION, EXECUTING THE FOLLOWING STEPS FOR EACH ONE:
2. DETERMINE AN OUTPUT QUANTITY
a. for the last operation, the work order quantity is the output
quantity
b. for each preceding operation, the output quantity is the input
quantity for the succeeding operation
3. DETERMINE AN INPUT QUANTITY
a. the start quantity is a quantity which when reduced by the scrap
rate and scrap amounts yields the input quantity for the
operation
4. DETERMINE MATERIAL REQUIREMENT
a. the absolute quantity of each material needed for an operation
is determined by multiplying the material requirement (a quantity
per unit of input or output) by the input or output quantity
determined in step 2 or 3, plus any scrap rate and/or amount
specified by the user; (this scrap rate/amount is independent of
the scrap rate/amount used in step 3)
The preferred embodiment of the scheduling system of the invention
determines the following information: (1) start date and finish
date for each operation, ignoring any capacity limitations; (2)
start date and finish date for each operation, taking into account
all capacity limitations; and (3) identity of the assigned main
resource, setup resource, and run resource. The assigned start
date, assigned finish date and assigned resource capacity are
displayed on a computer screen in a graphical format. In a
preferred embodiment of the invention, this information is
displayed in the form a bar graph.
FIG. 7 illustrates a preferred embodiment of a screen display
showing a schedule generated by the scheduling system of the
invention. The current month and days of the month are shown along
the x-axis in FIG. 7, and the various resources that are used in
the manufacturing process are shown along the y-axis. Only those
resources at which operations have been scheduled are included in
the screen display. The time frame that is displayed on the
computer screen may be varied. For example, the user may elect to
display all operations scheduled during a one week period or,
alternatively, all operations scheduled during a one month
period.
A preferred embodiment of the scheduling system of the invention is
capable of displaying more than one schedule on a single screen.
Multiple schedules may be created by the system based on different
resource capacity calendars defined by the user. For example, a
first schedule could be created based upon the availability of
three machine operators working in a particular shift, and a second
schedule could be created based upon the availability of five
machine operators in that shift. The scheduling advantage of this
increase in machine operator capacity could be evaluated by
displaying the first and second schedules on a computer screen
simultaneously.
While a preferred embodiment of the invention has been disclosed in
detail, it will be appreciated by those skilled in the art that
other embodiments of the invention within the scope of the
following claims may be developed based on the principles disclosed
herein.
* * * * *